1. Field of the Invention
This invention relates generally to a structural flange connection system and method, and more particularly to a structural flange connection system and method that utilizes structural flanges having a standard bolted connection and a mechanical bond effectively managing and assisting the retention of bolt preloads, and substantially eliminating movement between the flange faces, due to a reduction in the compromise of bolt preload due to flange face mismatch which can occur during the production of the flange connection joint.
2. Description of the Related Art
A great deal of interest is presently being shown in the development of alternative energy sources, particularly wind energy. New and more efficient wind turbine generators are being developed that are larger and produce more energy, but also produce more problems with regard to wind turbine tower design that combat the operational forces prevalent in normal operations of the wind turbine. Normal operations of the wind turbine create harmonic vibrations with each revolution of the turbine blades. Much research has been performed in tower operations in an effort to minimize vibration and stresses that ultimately cause metal fatigue and tower failure.
Wind turbine towers are typically constructed from rolled plate sections (cans), to which connection flanges are welded on each end, allowing a mechanical connection of the cans using bolts through the connection flange coupled to a nut on the opposite side of the flange, to create towers that range up to 300 feet in height. Each tower is made up of sections that can be shipped to the wind farm locations for erection. Typically two (2) to four (4) sections are utilized, depending on the height of the tower. Each section is attached to proximate sections by the use of connection flanges. These flanges can vary in diameter from approximately fifteen (15) feet at the bottom of the tower to approximately six (6) to eight (8) feet at the top of the tower where the turbine is attached. These flanges are bolted together using some 125 to 130 high strength bolts.
Because of the vibrations and stresses, these bolts are designed to be very tight with very high torque values. The high torque values generate high clamping loads in the bolted connection, which are far in excess of any fluctuating loads the joint experiences due to service loads, such as varying wind speed and direction, rated turbine power, rotating frequency, yaw angle and gyroscopic force generated by the change in the direction of the plane in which the blades of the turbine rotate. As the fluctuating loads are a small part of the load the joint experiences relative to the bolt clamping load, metal fatigue in the bolts is reduced as long as the clamping force is maintained. However, this clamping load can be compromised in service by the fact that manufacturing tolerances can result in the flange contact surfaces not being ideally plane. If a service load can cause the distance between the opposite flange faces of a bolted connection to settle or otherwise be reduced, the bolt in that connection un-stretches and the clamping load is reduced. As the clamping load is now a smaller part of the overall load experienced by the bolt, and the fluctuating load is a larger portion of that load, there is a potential for metal fatigue to be generated within the bolt of the connection, leading to its eventual failure. These forces that are inherent with normal wind tower turbine operations create the need for strict and costly maintenance procedures to ensure that the bolts at the flange joints are torqued to design specifications in order to maintain their design preload and re-establish any preload compromised due to a settling or deflection of the flange connection faces, and any damaged or failed due to metal fatigue are replaced in a timely manner.
Current practice dictates that the bolt tension of a flanged joint connection of a wind turbine tower must be checked after the first 500 hours of service. This is necessary because initially, the flange connection rests on dimensional imperfections of the flange faces, which generate areas of contact. These areas of contact can yield during initial loading and load cycling as the wind turbine tower is put into service. As these small deformations occur and the flange connections settle, dimensional variations between the mating flange faces can compromise the preload of the bolted connection. A maintenance operation of restoring the desired preload to the bolts by re-torquing them after the initial amount of service is required to assure that the bolt preload is restored, in case of such dimensional deflection.
If the flange faces are distorted by the manufacturing process, then a mismatch can occur. The net result of a mismatch is to form an area of higher stress in a given area of the flange. Depending on the installation and the location and degree of the distortion, this area of higher stress may yield, which could serve as a pivot or fulcrum about which shearing might be focused in order to affect a movement of the flanges at another location.
A potential for flange mismatch is generated by the nature of fabricating the tower sections. The vertical walls of the tower section are joined to the flanges, typically by welding, as bolting would require additional maintenance tasks in addition to the current flange bolt checks. As materials are welded, they are joined by the change of phase of the steel from a solid to a liquid, and back again. During this phase change, the material decreases in density and increases in volume and undergoes growth due to thermal expansion. As the material in a liquid phase now flows under any pressure, some material is extruded from areas under load. When the resulting melted joint re-solidifies, shrinkage results due to the extrusion of some of the original parent material and the contraction of the parent material as it cools. Depending on the levels of heat generated during the welding process, and the position and orientation of the weld, distortion of the flanges can occur. Once the flange is joined to the tower section, it becomes impractical to turn these faces back to true by a lathe operation, as the tower diameters at the flange are in excess of fifteen (15) feet.
Metal materials subject to cyclical loading are vulnerable to metal fatigue, which is the initiation and propagation of small cracks through the metal components under load. In steel materials, an endurance limit can be determined for the steel through testing, but is generally accepted to be one half the tensile strength. In other words, a load equivalent to less than 50% of the tensile strength under a complete reversal of loading would be considered an infinite life load for ferrous materials. Additionally, the infinite life is further modified by factors, such as application safety, the surface finish and geometric arrangement of the material, which reduce the allowable stress in the target design of the components. These practices are to ensure that an adequate margin of safety exists in the design's load carrying ability, while not over-sizing components needlessly, and impacting design realization efforts and costs.
Ideally, the flanges of the structural connection between towers sections are preloaded by the bolted connections, such that a compressive stress is generated under the bolt head and nut, which exceeds any fluctuating loads experienced by the tower connection under functional loads, including generator reactive torque, gyroscopic loads due to change of direction of the turbine rotational axis, and dynamic loads due to imbalance or resonance. The mating flange faces are loaded under the nut and bolt, with the loading being relaxed between bolted connections. Axial loads transmitted through the tower about an axis parallel with the vertical axis of the tower are resisted by the friction generated between the flange faces under the clamping load of the bolts by the coefficient of friction between the flanges.
If frictional force is reduced due to compromise of the bolt clamping force, or by excessive torque being transmitted through the structure, the bolts can possibly be brought into shear loading by the reduction of clearance between the bolt clearance holes and the bolts themselves. Once contact of a bolt with the wall of a clearance hole is made, any additional movement of the joint will result in shear loading within the bolt, effectively trying to shear the bolt across its profile (diameter). Furthermore, if this load is fluctuating in operation, it will have the potential of generating metal fatigue in the bolts through shear loading.
The primary mode of failure that exists in the structural connections of wind tower joints appears to be bolt failure by the compromise of bolt preload. The bolts begin to experience fluctuating loads and stresses once the bolt preload is reduced, and this fluctuating load leads to fatigue failure of the bolt. A friction fit alone between bolted flanges can be inadequate, especially given cyclical or repetitive loading in wind turbine towers.
It is therefore desirable to provide a structural flange connection system and method that utilizes structural flanges having a standard bolted connection and a mechanical bond that effectively manage and assist the retention of bolt preloads and substantially eliminate movement between the flange faces due to a reduction in the friction load being generated by the bolt clamping force in the flange connection.
It is further desirable to provide a structural flange connection system and method for manufacturing a wind turbine tower that lessens the damage created from the stresses associated with its intended use.
It is still further desirable to provide a structural flange connection system and method that provides equal distribution of external forces and minimizes the flow of stress forces through the structural flange connection.
It is yet further desirable to provide a structural flange connection system and method that uses a means of structural interface which tolerates dimensional variation and provides more consistent joint performance in terms of dimensional stability of the distance between the nut and the bolt head of a bolted joint connection.
It is yet further desirable to provide a structural flange connection system and method that utilizes a joint construction that allows for and accommodates significant deformations of the mated parts to allow a more uniform loading and seating of the joint, by design and not by the incidental potential variation of the joint in the manufacturing process.
It is yet further desirable to provide a structural flange connection system and method that uses a tapered pin between the flanges and in parallel connection with the bolts in the joint to create a pre-determined point of yielding of material to allow a consistent seat of the flange joint.
It is yet further desirable to provide a structural flange connection system and method that utilizes a tapered pin sized to create a predictable plastic deformation in flange material to seat the joint rather than allowing unpredictable flange face mismatch to dictate the characteristic performance of the flanged joint.
It is yet further desirable to provide a structural flange connection system and method having a proper taper of the pins in the joint to ensure that the joint captures the maximum compressive force generated at any given pin, by means of static frictional forces resulting from the pressure being generated by the compression of the pin and the expansion of its mating tapered hole, along with the coefficient of friction between the two materials.
It is yet further desirable to provide a structural flange connection system and method that effectively creates a preload of the joint in order to supplement the bolt preloading and help prevent the bolts from experiencing fluctuating stresses in the joint, resulting in longer joint life.